The invention relates in general to the field of quantum processing devices and operation thereof. In particular, it is directed to methods of operating quantum processing devices with quantum circuits (e.g., superconducting qubits), coupled to a frequency-tunable coupler, as well as related devices. Such methods aim at compensating for cross-talk between the quantum circuits.
Recent advances in quantum computing are making such a technology ever more relevant to industrial applications. Quantum computing makes direct use of quantum-mechanical phenomena, such as superposition and entanglement to perform operations on entangled quantum bits, i.e., states stored in quantum bits. Superconducting circuits are relatively easy to manufacture with current technologies and are thus promising candidates to further scale quantum information technologies.
Possible applications on quantum machines include the solving of hard optimization problems that are beyond the reach of classical algorithms. For example, quantum optimizations based on the variational principle are particularly appealing.
Quantum computing ideally needs a rapid and high-fidelity generation of entangled qubit states. For example, two-qubit gates are known, which are implemented with transmon (fixed-frequency) qubits, where the qubits are coupled via a frequency tunable coupler element. In contrast to other approaches, this implementation takes advantage of the high coherence of fixed-frequency qubits to generate entangled two qubit states with fidelities of more than 97%.
In particular, an architecture has been proposed, wherein two qubits are connected to a single tunable coupler, by David C. McKay and co-workers (Phys. Rev. Appl. 6, 064007 (2016)). This architecture allows transitions between two states of the two qubits to be parametrically driven, by modulating the tunable coupler energy at a frequency that corresponds to the difference of the energy between the two states.
More generally, quantum computing relies upon the ability to accurately control the states of the quantum circuits (e.g., qubits) making up the system. This involves being able to individually address each quantum bit in a well-defined manner as well as to create at least two qubit operations. When cross-talk between qubits is present, it is no longer possible to address one qubit without selectively and accurately knowing the state of the others. The dynamics of one qubit depends on the state of the others, which amounts to an unwanted two-qubit gate (controlled unitary operation). Thus, the presence of cross-talk impedes the operation of quantum computing devices.